Power transistors, such as double diffused metal-oxide semiconductor (DMOS) transistors are widely used in various power management applications, including used as switching elements in power supplies for industrial and consumer electronic devices. A power supply usually refers to a device or a system that supplies electrical or other types of energy to a load or a group of loads. A power supply comprising a power transistor switched ON and OFF at a relative high rate in response to control signals to convert a supply voltage into a desired output voltage is generally referred to as a switched-mode power supply, switching-mode power supply or SMPS. The available switching rate of the power transistor consists one of the factors that influent the power conversion efficiency, the size and the cost of the power supply. The higher the switching rate of the power transistor is available, the higher the switching frequency of the power supply may achieve, thereby, the smaller in size of the components for driving the power transistor, the lower in power consumption and the lower in manufacturing cost of the power supply.
A power transistor may generally comprise a gate, a drain, and a source lying in a semiconductor substrate. The gate regulates the conduction and blocking of a channel region in the substrate to control a current flow between the drain and the source. Take a lateral DMOS power transistor for example, the gate may overlie a gate dielectric having a thin portion over the channel region and part of a drift region, and a thick portion over an additional part of the drift region. The gate may be continuously formed over the surface of the gate dielectric, including over at least part of the thick portion of the gate dielectric. The thick portion of the gate dielectric can help to reduce electric field in the gate region, thereby supporting high drain-to-gate voltage, advantageously increasing the breakdown voltage of the DMOS power transistor. However, since the continuous gate extends over at least part of the thick portion of the gate dielectric, the drain-to-gate capacitance of the power transistor is increased, making the gate need more time to be charged and discharged for turning the transistor on and off, thereby slowing the switching rate of the power transistor.
Apart from a relatively high breakdown voltage, a relatively low on resistance, a relatively low drain-to-gate capacitance for supporting a relatively high switching rate, a thinner gate dielectric and/or an improved hot carrier lifetime are also desired for a power transistor.